Fortunately most of the highest detail textures and shaders on character models and weapons won't be needed beyond 5 to 10 meters.

I think the most important thing will be an engine that can scale to the appropriate levels of graphics cards and some exceptional net code. If they can manage those two things, I think the number of players will be relatively easy.

Look at some zombie games such as Left 4 Dead. It isn't the hundreds of character models that we will be seeing in Planetside, but it's a good example of how a well designed engine can accommodate a large number of moving entities without affecting performance much. The devs just have to start at 300 players on screen and work back from there, instead of 30.

I doubt you will ever have more than 30 people right next to you, especially not in any plausible combat situations, so hopefully distant view models will help a lot.

Net code is going to be the real killer here, but I have high hopes for it since that's really going to be an essential.